The invention relates to methods and apparatus of measuring the properties of paper from a moving paper web in a paper machine, in which measurement is carried out in the infrared region, most preferably in the wavelength range of 1 to 10 xcexcm.
U.S. Pat. No. 4,733,078 discloses an apparatus which directs a radiation beam to a target through a rotating filter wheel driven by a motor, the filters in the filter wheel emitting only a given wavelength of light at a time to the target. The light that has passed through the paper is detected at different wavelengths and the desired content of the substance is determined from the target on the basis of signals thus obtained. The problem in measuring a moving target is that the measurements of different wavelength channels are successive, since each filter illuminates a different point in the target. Before the following filter has had time to revolve in front of the light source, the target to be measured has slightly moved to a different point, and consequently the penetration point is not quite the same. In this case the proportional measurement, based on a reference wavelength, does not remove the gray factor variation caused by scattering, resulting in a significant error in measurements of non-homogenous, moving material, such as paper.
U.S. Pat. No. 3,641,349 discloses an apparatus in which a light beam, having passed through the paper sheet, is split into several parts. Each different part of the beam is detected by a separate detector, in front of which are disposed narrow-band filters, the signals being measured in parallel. Parallel measurement eliminates measuring noise caused by a non-homogenous moving target, but the number of channels is limited in this solution. The solution becomes cumbersome in applications requiring more than four channels, and its optical efficiency declines as the number of channels increases. Furthermore, because of the filters, resolution and central wavelength tolerances are quite bad, complicating the transfer of calibration from one device to another. The filters used also make it difficult to reach a sufficiently narrow band for the right wavelength, e.g. for measuring kaolin. Still further, lead sulphide detectors are extremely dependent on temperature, a different temperature in detectors disposed at different points causing significant errors in measurement.
WO 96/42010 discloses an apparatus for measuring the color and color-related properties of a moving web by reflectance measurements. This color measurement utilizes a black and white background behind the web, the reflected light beams being measured by two different sensors. The sensor uses a spectrograph which distributes the light beam to the detectors. The measured signals are transmitted serially from the detectors to a computer for further processing. However, serial processing is not always sufficiently efficient. Moreover, the solution of the publication cannot be utilized in applications in which the properties of paper are to be measured in the infrared region.
It is an object of the present invention to provide a method and an apparatus which avoid the above drawbacks.
The method of the invention is characterized by directing to the paper web an infrared beam modulated by a chopper, splitting the infrared beam which has passed through the paper web or reflected therefrom by a spectrograph into different wavelength components, measuring signals corresponding to the different wavelengths in parallel, detecting the signals using parallel detection synchronized to the chopper, and processing the signal in parallel and determining the properties of the paper on the basis of the wavelength components determined by the spectrograph.
The apparatus of the invention is characterized by comprising a radiation source for producing an infrared beam, a chopper for modulating the infrared beam, a spectrograph for splitting the infrared beam reflected from or passed through the paper web into different wavelength components, means synchronized to the chopper for parallel detection and processing of the wavelength components, and means for determining the properties of the paper on the basis of the different wavelength components split by the spectrograph.
An essential idea of the invention is to measure the properties of a moving paper web by directing an infrared beam modulated by a chopper to the paper, and splitting the beam which has passed through or reflected from the paper into different wavelengths by means of a spectrograph, measuring signals corresponding to the different wavelengths, detecting the signals using parallel detection synchronized to the chopper, and processing the signal in parallel. It is the idea of a preferred embodiment to split a light beam optically by a spectrograph into different wavelengths and to measure the signals of the different wavelengths from the spectrograph with separate detectors. The idea of another preferred embodiment is that the detectors employed are array detectors.
It is an advantage of the invention that because all measuring channels and reference channels are measured exactly simultaneously, the desired statistical accuracy is promptly reached when a non-homogenous rapidly moving sample is measured. Furthermore, detecting and processing the beam in parallel and continuously allows a narrow electric pass band for detector electronics, improving the signal to noise ratio of the detection and making the detection more immune to ambient light interference. Furthermore, measuring time can be utilized to a hundred percent for each channel, which also improves the signal to noise ratio of the detection. A spectrograph provides a high number of channels, good resolution, low central wavelength tolerances and repeatability of the wavelength scale from one device to another. The solution is also applicable to different numbers of wavelengths and to measurements of a continuous spectrum. A high number of wavelengths allows the effect of a change in sample temperature on the measurement to be compensated for, accurate measurements of binders, measurements of different binders, better immunity to interfering components and widening of the basis weight region of the paper to be measured by the use of different wavelength in different applications. In a device with a better wavelength resolution, reference channels can be brought nearer the measuring channel, resulting in e.g. better immunity to baseline bend. In an array detector the detector elements are in the same hermetic casing and of the same production lot, allowing a better immunity to be achieved to changes in ambient temperature than when several separate detectors are used.
In the present application, in addition to paper, the term xe2x80x9cpaperxe2x80x9d also refers to paperboard.